VOL: 98, ISSUE: 36, PAGE NO: 25
William Blows, PhD, BSc, RMN, RGN, RNT, OSEJ, is lecturer in biological sciences, St Bartholomew School of Nursing and Midwifery, City University, London
A lumbar puncture is a puncture into the subarachnoid space of the spinal cord to obtain cerebrospinal fluid (CSF) for clinical investigation, to remove excess fluid or to inject medication.
The three coverings of the brain and spinal cord, known as the meninges, are separated by small spaces. The outer meninx, the dura mater, is the strongest layer and forms attachments to the skull in later life. The dura mater is in two layers, and between them is a space containing tissue fluid, blood vessels and venous sinuses.
Beneath the inner of the two layers is a small subdural space. The other side of this space is the middle layer, the arachnoid mater that has a more delicate structure, like a spider’s web. Below this is the subarachnoid space that is filled with fluid and contains the arachnoid trabeculae that link the arachnoid with the pia below. The innermost third layer, which rests on the brain surface, is the delicate and vascular pia mater (Fig 1).
The CSF in the subarachnoid space is produced in the third and fourth ventricles of the brain and then passes via canals to the subarachnoid space. It flows down the spinal cord and returns back within the spinal canal to the brain.
Cerebrospinal fluid is derived from blood plasma. It is secreted from capillaries within the choroid plexus, which are extensions of the vascular pia mater that dip down into the roof of the third and fourth ventricles (Blows, 2000). The fluid fills the entire ventricular system inside the brain - the two lateral ventricles and the third and fourth ventricles - as well as the subarachnoid space and the central canal of the spinal cord.
The flow is caused by the action of cilia extending from cells that line these spaces. These provide a beating motion that sweeps the fluid along. The rate of CSF production must be met by an equal rate of CSF reabsorption to prevent any build-up of fluid in the brain and spinal cord. The fluid is reabsorbed by arachnoid villi which return it to the venous blood before it leaves the head (Blows, 2000).
Normal CSF is mostly water. It also contains:
- Protein (15-45mg per 100ml);
- Glucose (40-80mg per 100ml);
- Lactate (1.1-l.9mmol per litre);
- Lymphocytes (0-5 cells per mm3).
These values, and the concentration of potassium, calcium and bicarbonate in CSF, are lower than the equivalent values found in blood plasma. CSF has a pH of about 7.3, which is just slightly more acidic than blood plasma that has a pH of 7.4.
The total volume of CSF is 130-150ml at any given time. This volume is replaced three to four times a day at roughly eight-hourly intervals. About 500ml of CSF is produced every day. Normal CSF pressure measured with a manometer is 60-160mm of water.
CSF has a number of functions:
- It acts as a shock absorber to the central nervous system, protecting it against trauma;
- It provides buoyancy, for example, the apparent weight of the brain is nearly 30 times lighter than its dead weight as a result of it ‘floating’ in CSF;
- Changes in the rate of production or absorption of CSF can help to compensate for pressure changes in the skull. These may occur when a space occupying lesion (SOL) develops. The differential in production and absorption of CSF, along with changes in blood flow through the brain, help to prevent any major increase in intracranial pressure as an SOL grows in size. Such compensation can be effective only for a limited time, depending on the rate of growth of the SOL;
- CSF is an exchange medium for the brain, helping it to keep the biochemical environment stable and removing waste metabolites for excretion.
This is usually a safe procedure undertaken to acquire a sample of CSF for analysis, and is also sometimes done to measure CSF pressure or to introduce drugs into the CSF, a procedure known as intrathecal injection. The insertion of a needle under local anaesthetic requires careful positioning to prevent injury to the spinal cord.
Since the spinal cord ends as a solid structure around the level of the second lumbar vertebra (L2) the insertion of a needle must be below this point, usually between L3 and L4 (Fig 2). The spinal cord continues below L2 down into the sacrum as many separate strands of nerve pathways, the cordae equina, bathed in CSF.
Putting a needle into the spaces between the strands to collect fluid is much safer than taking the risk of hitting the solid cord higher up the spine. The spinal vertebrae are held together by ligaments. Those penetrated in a lumbar puncture are the interspinous ligaments (which bind adjacent spinous processes together) and the ligamentum flavum (which binds adjacent vertebral laminae together and, in so doing, lines the posterior wall of the spinal canal).
Lumbar puncture must be carried out as a sterile procedure, with full aseptic precautions to prevent the introduction of outside organisms into the spinal canal and the contamination of the specimens collected. The patient must be either:
- Sitting up, bent over a table;
- Lying on one side (a lateral recumbent position), with knees drawn up towards the chest and the back exposed.
The latter position curves the spine anteriorly, providing the maximum room between the vertebral processes to allow the insertion of the needle. It may be necessary for the nurse to assist the patient into this position, and give them support to maintain it during the procedure.
Placing the patient towards the edge of the bed gives the doctor easy access. The correct site is determined by imagining a line drawn from one iliac crest to the other (anterosuperior iliac spines), and identifying where this line crosses the lumbar spine. This is around the level of L3, and can be confirmed by counting the lumbar vertebral neural spines that can be seen and felt extending into the skin.
The lumbar area is cleaned with an appropriate antiseptic before local anaesthetic is instilled into the skin and deeper tissues, and given a few minutes to act. The needle is then inserted into the intravertebral space between L3 and L4. It will penetrate the skin, then the subcutaneous tissues, the interspinous ligaments and the ligamentum flavum before passing through the dura and arachnoid maters and entering the subarachnoid space and the CSF.
CSF pressure can be measured with a manometer. When this is removed, the fluid is allowed to drip into sterile containers that are sealed and labelled. The samples are used for cell counts, cytology, glucose and protein measurements and sometimes immunoglobulin studies. They may also be taken for bacteriological or viral tests, and for biochemical analysis.
After the procedure has been completed, the needle is removed and a small local dressing applied to absorb any CSF or blood leakage. The patient should remain flat for six to 12 hours because headache, which is common after lumbar puncture, is aggravated by sitting up. Headaches may be severe and may require treatment with analgesia (Hickey, 1997).
Lumbar punctures may also be performed to introduce a contrast radiopaque medium into the CSF to provide radiographic pictures of the spine and spinal cord, or myelograms. These are used in the diagnosis of spinal lesions, such as a prolapsed intravertebral disc, which interfere with CSF flow or spinal cord function. Myelograms help in planning surgery by isolating the level of the lesion and selecting the most suitable part of the spine for operation.
The contrast medium must not be allowed to enter the skull because it can cause seizures. Natural CSF flow will cause the contrast medium to diffuse up the spine as desired, but to prevent seizures the patient’s head must be kept raised about 30° from the horizontal. This must be maintained for up to 24 hours afterwards, depending on the patient’s condition and local policy procedures. Patients should be well-hydrated for myelograms and standard lumbar punctures, and should be encouraged to drink well before and after the procedures.
Normal CSF is a clear watery fluid, but the presence of cells, protein or blood can alter its appearance. Cells in larger than normal numbers cause turbidity. These may be both lymphocyte and pus cells, which indicate an infection. Lumbar punctures are carried out to isolate the organism involved in meningitis, and to give intrathecal antibiotics.
Fresh blood may be visible in CSF because of bleeding at the puncture site. This may be owing to injury to a vertebral vein by the needle, and the sample will become clear as it continues to drip from the needle (Sharief, 2000). Continuous blood contamination indicates bleeding into the subarachnoid space, usually the result of a very recently (within hours) ruptured aneurysm somewhere in the head, possibly in the circle of Willis (Blows, 2001).
A continual yellow stain to the fluid is known as xanthochromia, and is usually due to a subarachnoid bleed that has had time for the red cells to settle, probably after 24 hours, leaving other plasma components such as protein and other cells suspended in the CSF. High protein counts may be due to infections of the meninges or brain, or can occur as a result of brain tumours or multiple sclerosis (Sharief, 2000). Other changes of CSF content as a result of central nervous system infections are reduced or absent glucose levels and a disturbance in lactate levels. These are higher in bacterial than in viral meningitis.
Increased CSF pressure is indicative of raised intracranial pressure due to SOLs such as tumours or intracranial bleeds. It is also seen in hydrocephalus, where an accumulation of CSF occurs in the ventricles of children’s brains because CSF flow and drainage is blocked (Blows, 2001).